Autostereoscopic optical apparatus for viewing a stereoscopic virtual image

ABSTRACT

An apparatus for forming an image on a curved diffusive surface ( 32 ) comprises an image source ( 94 ) for providing image-bearing light along an axis, a relay lens ( 54 ) for directing the image-bearing light toward the curved diffusive surface ( 32 ), and, a field lens ( 120 ) for redirecting off-axis image-bearing light toward the center of curvature of the curved diffusive surface ( 32 ).

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional of application Ser. No. 10/465,503, filedJun. 19, 2003.

FIELD OF THE INVENTION

[0002] This invention generally relates to projection apparatus and moreparticularly relates to an apparatus and method for forming a curvedintermediate image from a substantially flat image source.

BACKGROUND OF THE INVENTION

[0003] In conventional projection apparatus, an image, provided from anessentially flat image-forming surface, is projected onto an essentiallyflat display surface. In film-based projection, for example, light istransmitted through a flat piece of film for projection onto a flatmovie screen. Digital image-forming devices, such as transmissive andreflective LCDs and digital micromirror devices (DMDs) similarly producean image on a flat surface. This allows projection apparatus employingthese devices to use output optics that are similar to the projectionoptics employed in film-based projectors.

[0004] A number of optical systems, however, form images using curvedsurfaces, particularly optical systems of the immersive type that aredesigned to provide a wide field of view. One example system of thistype is disclosed in commonly assigned U.S. Pat. No. 6,416,181 (Kessleret al.) and U.S. Pat. No. 6,522,474 (Cobb et al.), both incorporatedherein by reference, herein referred to as either the '181 or '474patent. In an autostereoscopic imaging apparatus 10 as described in the'181 disclosure and as shown in FIG. 1, a curved mirror 24 is employed,in combination with a beamsplitter 16 for providing an autostereoscopicvirtual image to a viewer 12 at left and right viewing pupils 141 and 14r. For each viewing pupil 141 and 14 r, an image generation system 70provides an initial intermediate curved image that is then projectedthrough a ball lens assembly 30 in order to form a left or rightintermediate curved image at a focal plane of curved mirror 24.

[0005] The monocentric optical apparatus of the '181 disclosure providesautostereoscopic imaging with large viewing pupils, a very wide field ofview, and minimal aberration. In order to provide this type of imagingand take advantage of the inherent benefits of monocentric projection,the apparatus of the '181 disclosure, given its source image formed on aflat surface, must form an intermediate image having a suitablecurvature. Referring to FIG. 2, there is shown, extracted from the moredetailed description of the '181 disclosure, a portion of an imagegeneration system 70 for providing an intermediate curved image 80 forprojection, for either eye, in autostereoscopic imaging apparatus 10.Here, an image generator 74 provides a source image from an image source94, where image source 94 has a flat surface, such as from a reflectiveLCD. A relay lens 54 directs light from image generator 74 onto adiffusing element 32, so that a curved intermediate image 76 is formedon a diffusive surface 40. Ball lens assembly 30, cooperating withbeamsplitter 16, projects curved intermediate image 76 toward a frontfocal surface 22 of a curved mirror 24 to form intermediate curved image80. Curved mirror 24 then provides a virtual image of intermediatecurved image 80 at viewing pupil 14.

[0006] Using the overall arrangement of FIG. 2, image source 94 can beany of a number of image sources that emit light, such as a transmissiveor reflective LCD spatial light modulators, a digital micromirror device(DMD) spatial light modulator, a CRT, or an OLED or PLED device, forexample. Significantly, the image formed on image source 94 issubstantially flat. There may be some slight curvature to this image,such as would be provided by a CRT; however, the arrangement of FIG. 2works well when image source 94 is flat and shows how intermediate image80 can be formed having the needed curvature. Since most image displaydevices form a flat image, there is, then, no need for modification tooff-the-shelf display components with this arrangement.

[0007] As the '181 disclosure points out, forming an intermediate imageon a diffusive surface helps to overcome limitations imposed by theLaGrange invariant. A product of the size of the emissive device and thenumerical aperture, the LaGrange invariant determines output brightnessand is an important consideration for matching the output of one opticalsystem with the input of another. Use of the diffuser with the '181apparatus is necessary because the image-forming device, typically areflective LCD or other spatial light modulator, is a relatively smallemissive device, measuring typically no more than about 1 inch square.Referring again to FIGS. 1 and 2 and to the '181 disclosure, in order tomaximize the light output from image generator 74, it is necessary toprovide a large angle of emitted light, using diffusing element 32, inorder to adequately fill left and right viewing pupil 141 and 14 r.Diffusive surface 40 is shaped to provide curved intermediate image 76having the desired curvature for the projection optical system.

[0008] While use of a diffusing element 32 provides a workable solutionfor forming a curved image, there are some drawbacks to projecting animage onto a diffusive component. In order to understand drawbacks withparticular impact upon autostereoscopic imaging apparatus 10, it isinstructive to consider how ball lens assembly 30 operates. Referring toFIG. 3a, there is shown the concentric arrangement and optical behaviorof a ball lens assembly 30 for directing light from a curved image 50. Acentral spherical lens 46 is disposed between meniscus lenses 42 and 44.Central spherical lens 46 and meniscus lenses 42 and 44 have indices ofrefraction and dispersion characteristics intended to minimize on-axisspherical and chromatic aberration, as is well known in the opticaldesign arts. An aperture stop 48 defines a ball lens pupil 106 withinball lens assembly 30. Aperture stop 48 need not be a physical stop, butmay alternately be implemented using optical effects such as totalinternal reflection. In terms of the optics path, aperture stop 48serves to define an entrance pupil and an exit pupil for ball lensassembly 30.

[0009] In most embodiments, meniscus lenses 42 and 44 are selected toreduce image aberration and to optimize image quality for the projectedimage projected. It must be noted that ball lens assembly 30 couldcomprise any number of arrangements of support lenses surroundingcentral spherical lens 46. Surfaces of these support lenses, howevermany are employed, would share a common center of curvature withC_(ball), the center of curvature of central spherical lens 46.Moreover, the refractive materials used for lens components of ball lensassembly 30 could be varied, within the scope of the present invention.For example, in addition to standard glass lenses, central sphericallens 46 could comprise a plastic, an oil or other liquid substance, orany other refractive material chosen for the requirements of theapplication. Meniscus lenses 42 and 44, and any other additional supportlenses in ball lens assembly 30, could be made of glass, plastic,enclosed liquids, or other suitable refractive materials, all within thescope of the present invention. In its simplest embodiment, ball lensassembly 30 could simply comprise a single central spherical lens 46,without additional supporting refractive components.

[0010] In ideal operation, curved image 50 shares the same center ofcurvature C_(ball) as ball lens assembly 30. When arranged in thisfashion, light from any point on curved image 50 is imaged with minimalaberration, as is represented in FIG. 3a.

[0011] The inherent advantages of a ball lens can be exploited using amodified design that employs a partial ball lens segment, such as usingan hemisphere combined with a folding mirror, as is shown in thecross-sectional ray diagram of FIG. 3b and described in the '474 patent.In FIG. 3b, a hemispheric lens assembly 60 comprises a hemisphericcentral lens 66, one or more optional meniscus lenses 42, and areflective surface 62 along the meridional plane of the hemisphere.Reflective surface 62 may be formed over the full surface of themeridional plane or may be formed only along a portion of this surface.As shown in FIG. 3b, hemispheric lens assembly 60 forms, from curvedimage 50 as its object, a curved image 64, folding the optical path atthe same time. This arrangement can have advantages, for example, wherespace for optical components is constrained.

[0012] Referring to FIG. 4, there are shown ray traces of principal raysfor projected light from image generation system 70. Light along opticalaxis O is incident to diffusing element 32 at a normal angle. As theprojected light becomes off-axis, the incident angle onto diffusingelement 32 also changes. At the edge of the projected field, as shown atan off-axis ray 200, the incident angle varies significantly fromnormal. This has a number of undesirable effects, as is shown in FIGS.5a and 5 b. Both on-axis light, as shown in FIG. 5a, and off-axis light,as shown in FIG. 5b, strike diffusing element 32 and are spread over awide range of angles. The problems of most interest include thefollowing related behavior:

[0013] (i) Hot spot. As a comparison of the clusters of rays in FIGS. 5aand 5 b suggests, a higher percentage of on-axis light is provided toball lens assembly 30 than of off-axis light; the result is a hot spotalong optical axis O. Solutions for minimizing this effect includemechanical dithering of diffusing element 32; however, this type ofsolution adds cost for dithering components and requires further designcompensation for high-frequency vibration effects.

[0014] (ii) Only a fraction of off-axis light reaches central sphericallens 46 for projection. Other light may be scattered throughout theoptical system, reducing contrast.

[0015] As FIGS. 5a and 5 b show, providing diffused light over a wideangle is necessary in order to obtain at least some level of brightnessfrom off-axis light. At the same time, however, wide angle diffusionreduces contrast due to stray light leakage. Thus, even given someamount of curvature, diffusing element 32 is constrained with respect toefficiency. The requirements for large angle diffusion add complexityand cost to the design of diffusing element 32. Thus, although the useof curved diffusing element 32 helps to surmount LaGrange invariantlimitations, there is still room for improvement in contrast andbrightness and a need to minimize or eliminate any hot spot effects. Ofparticular value would be methods that provide these performanceimprovements at low cost and with minimum complexity.

[0016] Thus, it can be seen that there is a need for an imagingsubsystem that provides, from a substantially flat image source, acurved image having high brightness as an intermediate image forprojection and display apparatus.

SUMMARY OF THE INVENTION

[0017] It is an object of the present invention to provide an apparatusand method for forming a curved image from a substantially flat imagesource. With this object in mind, the present invention provides anapparatus for forming an image on a curved diffusive surface,comprising:

[0018] (a) an image source for providing image-bearing light along anoptical axis;

[0019] (b) a relay lens for directing the image-bearing light toward thecurved diffusive surface; and

[0020] (c) a field lens for redirecting off-axis image-bearing lighttoward the center of curvature of the curved diffusive surface.

[0021] It is a feature of the present invention that it employs a fieldlens to redirect projected light towards a pupil, allowing subsequentprojection of the image with its curvature preserved.

[0022] It is an advantage of the present invention that it providesimproved contrast for an image projected onto a curved diffusivesurface.

[0023] It is a further advantage of the present invention that itminimizes the characteristic hot spot along the optical axis that canresult using conventional optical methods.

[0024] It is yet a further advantage of the present invention that iteases wide angle performance requirements of a curved diffusive surface.

[0025] These and other objects, features, and advantages of the presentinvention will become apparent to those skilled in the art upon areading of the following detailed description when taken in conjunctionwith the drawings wherein there is shown and described an illustrativeembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] While the specification concludes with claims particularlypointing out and distinctly claiming the subject matter of the presentinvention, it is believed that the invention will be better understoodfrom the following description when taken in conjunction with theaccompanying drawings, wherein:

[0027]FIG. 1 is a perspective view showing an autostereoscopic opticalapparatus that provides a curved image for projection;

[0028]FIG. 2 is a schematic diagram showing a prior art arrangement ofoptical components for providing a curved image formed on a diffusiveelement and projected using a ball lens;

[0029]FIG. 3a is a cutaway side view showing components and overalloptical behavior of a ball lens assembly;

[0030]FIG. 3b is a cutaway side view showing components and opticalbehavior of an hemispheric lens having a reflective surface;

[0031]FIG. 4 is a side view diagram showing the path of principal raysprojected onto a diffusive element;

[0032]FIGS. 5a and 5 b show wide angle rays directed for projection by aball lens assembly, for on-axis and off-axis illumination, respectively;

[0033]FIG. 6 is a side view diagram showing the path of projected raysdirected by a field lens, in one embodiment of the present invention;

[0034]FIGS. 7a and 7 b show narrower angle rays directed for projectionby a ball lens assembly, for on-axis and off-axis illumination,respectively, using the apparatus of the present invention;

[0035]FIG. 8 is a side view diagram showing the path of projected raysdirected by a field lens, in an alternate embodiment of the presentinvention;

[0036]FIGS. 9a and 9 b show narrower angle rays directed for projectionby a ball lens assembly, for on-axis and off-axis illumination,respectively, using the curved Fresnel apparatus of the presentinvention;

[0037]FIG. 10 shows a magnified side view using a Fresnel field lens anda diffuser sheet;

[0038]FIG. 11 shows a perspective view of a compact autostereoscopicimaging system employing the projection apparatus of the presentinvention using a pair of hemispheric lenses; and

[0039]FIG. 12 shows a perspective view of a compact autostereoscopicimaging system employing the projection apparatus of the presentinvention using one hemispheric lens paired with one spherical balllens.

DETAILED DESCRIPTION OF THE INVENTION

[0040] The present description is directed in particular to elementsforming part of, or cooperating more directly with, apparatus inaccordance with the invention. It is to be understood that elements notspecifically shown or described may take various forms well known tothose skilled in the art.

[0041] First Embodiment

[0042] Referring to FIG. 6, there is shown a first embodiment of thepresent invention. Image source 94 provides light from its flat sourceimage to relay lens 54 in image generation system 70. The light outputfrom relay lens 54 fans outward from optical axis O. A field lens 120redirects the off-axis light through diffusing element 32 toward thecenter of curvature C of diffusing element 32. In contrast with theprior art approach shown in FIG. 4, the optical arrangement of FIG. 6directs off-axis rays at a normal to the surface of diffusing element32, minimizing the optical effects that can cause hot spots, as wasdescribed hereinabove. Moreover, by directing the light towards thecenter of curvature C of diffusing element 32, field lens 120 providesan optimal arrangement for using ball lens assembly 30. Referring toFIGS. 7a and 7 b, there are shown the paths of light rays for bothon-axis and off-axis light incident on diffusing element 32. As can beseen by comparison with the prior art solution shown in FIGS. 5a and 5b, an arrangement using field lens 120 requires less angular spread ofthe incident light. For example, the prior art approach of FIGS. 4, 5a,and 5 b typically requires a half-angle of diffusive light of about 30degrees. In comparison, the use of field lens 120 as shown in FIGS. 6,7a, and 7 b reduces the needed half-angle of diffusive light to about 12degrees.

[0043] As is shown in FIG. 6, field lens 120 may consist of a number ofdifferent lens elements. One or more of the component lenses of fieldlens 120 may be aspherical.

[0044] Second Embodiment

[0045] The embodiment of FIG. 6 employs conventional lens components asfield lens 120. However, while there are advantages for system opticalperformance, the use of conventional glass or plastic optics can addbulk and weight to the overall imaging system. In response to the needto reduce size and weight, FIGS. 8, 9a, and 9 b show an alternativeembodiment using a curved Fresnel lens 124 and a diffusion film 126.Comparing the optical arrangement of FIG. 8 with that of FIG. 6, it canreadily be seen that an embodiment using curved Fresnel lens 124 anddiffusion film 126 would have advantages for reduced size and weight.FIGS. 9a and 9 b show the angular spread of diffused light when usingcurved Fresnel lens 124 and diffusion film 126. Relative to overallperformance and contrast, the conventional optics employed in theapproach of FIG. 6 are slightly advantaged over the use of curvedFresnel lens 124 and diffusion film 126 in FIG. 8. However, for manyapplications, a slight performance compromise may be acceptable inconsideration of the significant reduction in weight and size.

[0046] In practice, curved Fresnel lens 124 and diffusion film 126 canbe packaged closely together, as is shown in the close-up side view ofFIG. 10. A few microns of air-space might be provided between curvedFresnel lens 124 and diffusion film 126.

[0047] Custom optical suppliers provide Fresnel lens components in arange of types and shapes, including components having curved surfaces.A few examples of component suppliers are the following: ReflexiteDisplay Optics, Rochester, N.Y. and Fresnel Technologies Inc., FortWorth, Tex.

[0048] Diffusing element 32 is also curved and may be provided in anumber of ways. For example, diffusing element 32 may be a fiber opticfaceplate or a treated glass or plastic component, having a ground orchemically treated surface. Alternatively, holographic diffusivesurfaces and diffusion films 126 could be used. Holographic diffusivesurfaces and diffusion films are available from a number of suppliers,such as Reflexite Display Optics and Physical Optics Corporation,Torrance, Calif.

[0049] Embodiment Within Autostereoscopic System

[0050] Referring to FIG. 11, there is shown a perspective view ofautostereoscopic imaging apparatus 10 using image generation system 70that employs field lens 120. For compact packaging, each imagegeneration system 70 has a turning mirror 34 for providing modulatedlight from relay lens 54 to field lens 120. In the embodiment of FIG.11, both left and right image paths employ a hemispheric lens assembly60, as was shown in FIG. 3b.

[0051] Referring to FIG. 12, an alternate embodiment forautostereoscopic imaging apparatus 10 is shown. In this arrangement, oneimage generation system 70 (for left viewing pupil 141) uses ahemispheric lens assembly 60, the other image generation 70 (for rightviewing pupil 14 r) employs a spherical ball lens assembly 30. Bypairing these two different types of ball lenses in a single system, thearrangement of FIG. 12 offers additional opportunities for compactpackaging of autostereoscopic imaging apparatus 10.

[0052] The invention has been described in detail with particularreference to certain preferred embodiments thereof, but it will beunderstood that variations and modifications can be effected within thescope of the invention as described above, and as noted in the appendedclaims, by a person of ordinary skill in the art without departing fromthe scope of the invention. For example, any suitable field lensarrangement could be used for redirecting off-axis light toward centerof curvature C of diffusing element 32.

[0053] Thus, what is provided is an apparatus and method for forming acurved image from a substantially flat image source and anautostereoscopic display using that apparatus.

Parts List

[0054]10 Autostereoscopic imaging apparatus

[0055]12 Viewer

[0056]14 Viewing pupil

[0057]14 l Viewing pupil, left

[0058]14 r Viewing pupil, right

[0059]16 Beamsplitter

[0060]22 Front focal surface

[0061]24 Curved mirror

[0062]30 Ball lens assembly

[0063]32 Diffusing element

[0064]34 Turning mirror

[0065]40 Diffusive surface

[0066]42 Meniscus lens

[0067]44 Meniscus lens

[0068]46 Central spherical lens

[0069]48 Aperture stop

[0070]50 Curved image

[0071]54 Relay lens

[0072]60 Hemispheric lens assembly

[0073]62 Reflective surface

[0074]64 Curved image

[0075]66 Hemispheric central lens

[0076]70 Image generation system

[0077]74 Image generator

[0078]76 Intermediate image

[0079]80 Intermediate curved image

[0080]94 Image source

[0081]106 Ball lens pupil

[0082]120 Field lens

[0083]124 Curved Fresnel lens

[0084]126 Diffusion film

[0085]200 Off-axis ray

What is claimed is:
 1. An autostereoscopic optical apparatus for viewinga stereoscopic virtual image, said stereoscopic virtual imagecomprising: a left image for viewing by an observer at a left viewingpupil, said left image formed by a left image generation system thatprovides a left curved intermediate image that is projected toward acurved mirror by a left ball lens segment; a right image viewing by saidobserver at a right viewing pupil, said right image formed by a rightimage generation system that provides a right curved intermediate imagethat is projected toward said curved mirror by a right ball lenssegment; each said image generation system comprising: (a) an imagesource for providing image-bearing light along an axis; (b) a relay lensfor directing said image-bearing light toward a curved diffusivesurface; (c) a field lens for redirecting off-axis image-bearing lighttoward the center of curvature of said curved diffusive surface; andeach said image generation system thereby forming said curvedintermediate image on its respective said curved diffusive surface. 2.An autostereoscopic optical apparatus according to claim 1 wherein, forsaid left image generation system, the center of curvature of saidcurved diffusive surface is substantially coincident with the center ofcurvature of said left ball lens segment.
 3. An autostereoscopic opticalapparatus according to claim 1 wherein, for said right image generationsystem, the center of curvature of said curved diffusive surface issubstantially coincident with the center of curvature of said right balllens segment.
 4. An autostereoscopic optical apparatus according toclaim 1 wherein said left ball lens segment comprises a centralspherical lens.
 5. An autostereoscopic optical apparatus according toclaim 1 wherein said left ball lens segment comprises at least onemeniscus lens.
 6. An autostereoscopic optical apparatus according toclaim 1 wherein at least one of said left and right ball lens segmentscomprises an hemispheric lens and a reflective surface.
 7. An apparatusfor forming an image according to claim 1 wherein said image source is aspatial image modulator selected from the group consisting of atransmissive LCD, a reflective LCD, and a digital micromirror device. 8.An apparatus for forming an image according to claim 1 wherein thecenter of curvature of said curved diffusive surface is on said axis. 9.An apparatus for forming an image according to claim 1 wherein saidfield lens is a Fresnel lens.
 10. An apparatus for forming an imageaccording to claim 9 wherein said Fresnel lens is curved.
 11. Anapparatus for forming an image according to claim 1 wherein said fieldlens comprises an aspheric surface.
 12. An apparatus for forming animage according to claim 1 wherein said curved diffusive surface is aholographic surface.
 13. An apparatus for forming an image according toclaim 1 wherein said curved diffusive surface comprises a treated glasssurface.
 14. An apparatus for forming an image according to claim 1wherein said curved diffusive surface is taken from the group consistingof a diffusion film and a fiber optic faceplate.